This invention relates to molds, and more specifically to molds for producing such molded articles as phonograph records, compact disks, photomagnetic disks, photo disks, and laser disks. The invention can provide molds of outstandingly long service life. The term "molds" as used herein also encompasses stampers that serve as negatives for the above-mentioned disks.
Recorded media, including phonograph records and audio-visual disks, are made by charging plastic material into the cavity of a mold supporting a stamper (negative) therein, and applying a pressure so as to mold the charge into a desired medium shape while, at the same time, reproducing the surface pattern of the stamper as a reverse impression on the molded surface. A typical example of such a mold is illustrated in FIG. 1. An injection mold for producing laser disks or the like, it comprises a movable mold half 2 and a fixed mold half 5, in such a relation that when the movable mold half is in its closing position a mold cavity 7 is formed. The surface 8 of the movable mold half 2 to face the cavity 7 is mirror polished and supports a metal stamper 1 in the form of a nickel-plated sheet metal. The stamper is securely held in place along the periphery by an outer ring member 4. This member 4 also constitutes the surrounding wall of the cavity. FIG. 1 shows the mold closed, defining the cavity 7 inside. Resin under a given molding pressure is fed through a feed nozzle 3 and the gate 6 of a gate member 12 into the cavity 7, where it is molded to a desired shape. The movable mold half 2 used is made of steel, quenched and hard plated on the surface 8, with a high-precision polish. Thus the mold half is polished smooth enough to permit the stamper to slide over the mold surface 8 upon its contraction or expansion with heat. For example, when the molten resin temperature is 360.degree. C., the temperature of the movable mold half surface 8 is 100.degree. C., and the resin pressure is 400 kg/cm.sup.2, the stamper 1 is heated to 360.degree. C. on the side contacting the melt and to 100.degree. C. on the back side, under a pressure equivalent to the resin pressure. Under these conditions, the stamper is caused to move along the surface 8 by the heat and pressure. Repetition of the injection molding cycle with such a mold gradually injures the stamper 1, shot after shot, with the friction involved. The stamper eventually crazes and transfers the craze marks onto the surface of the molded object. The present inventors have observed the development of such defects on the stamper side facing the mounting surface of the mold. The stamper is disk-shaped and restrained at the center so that a certain degree of turning is allowed. Consequently, the localized motion of the stamper due to expansion and shrinkage during molding is the resultant of two motions, radial and rotational, and the more distant from the center the greater. The size of crazes actually measured along the outer periphery of such a stamper was up to about 0.1 mm wide (in the radial direction) and about 1 mm long (in the turning direction). The crazes grew deep with the repeated use of the stamper.
This problem is alleviated to some extent by coating the surface 8 on the cavity side of the stamper with a hard-wearing film of TiN or the like. Adequate wear resistance and low frictional properties are not attained, however.
Another problem associated with the mold of the character is the wear of the gate cut portion. A disk or other molded part, after the solidification of the resin, is formed with a center hole by the shearing action of the edge of a gate cut portion 10 of a gate cut member 9 moved rightward as viewed in FIG. 1, in cooperation with the edge of the hole of the fixed mold half 5. The consequent friction gradually wears down the gate cut portion 10 and the matching portion of the fixed mold half, making the edges dull and eventually chipping them off.
Yet another problem that the mold poses is the corrosion of the air vent. Usually, to facilitate the injection of resin into the mold cavity, an air vent 11 is provided as a gap of about 10 to 20 microns between the fixed mold half 5 and the outer ring member 4. As resin is injected rapidly at elevated pressure, entrapped air is forced out through the air vent 11, while corroding the surrounding wall surface of the passageway. The corrosion is attributed to the attacks by the monomers that resulted from the decomposition of the resin, by the moisture content in the hot air, and by the air escaping at high speed.
A wear problem is further presented by the inner surface of the mold cavity on which molten resin at elevated temperature and pressure usually solidifies and shrinks.
Among the important requirements the stamper and other mold parts must meet are good mold releasability with respect to the molded resin and high surface hardness. In the case of a photo disk, for example, the disk surface is formed with minute irregularities about 0.1 .mu.m high. Only a slight residue of resin left on the stamper surface could mar the stamping function, reduce the accuracy of reproduction, and shorten the life of the stamper.
In order to overcome these problems a number of methods have been proposed to coat the stamper and other mold parts with a hard, highly mold-releasing film. The countermeasures are roughly classified into two categories: (1) coating by surface oxidation, carbonizing, nitriding, or the like, and (2) formation of an organic polymer film. The formation of a coating film by surface oxidation or the like was taught by Japanese Patent Application Public Disclosure Nos. 130203/1982, 124046/1984, 218646/1984, 173734/1985, 272362/1986, and others. According to these inventions, mold release characteristics are improved by inactivating the surface through oxidation, sulfurization, carbonization, or nitriding by liquid- or vapor-phase treatment, or both mold releasability and durability are improved by increasing the hardness with a coat of another metal oxide (e.g., SnO.sub.2, TiO.sub.2, or ZrO.sub.2) formed by sputtering, or with a surface oxidized by oxygen-plasma treatment.
However, coated films of oxides, such as metal oxides, and nitrides seldom attain adequate durability, because they tend to be locally cracked by repetitive subjection to forces at shots of injection molding. Coats of this character do not provide satisfactory mold releasability yet and cannot be said to be durable.
Forming an organic polymer film on the surface of the stamper and other mold parts was disclosed by Japanese Patent Application Publication No. 10763/1983 and Japanese Patent Application Public Disclosure Nos. 209834/1984, 218821/1984 and others. These processes involve the polymerization on the stamper surface of a fluorine compound, aliphatic halogen compound, aromatic halogen compound or the like converted to a plasma or activated by glow discharge, sputtering, or other treatment. Of those stampers, one coated with a fluorine polymer film exhibits outstanding mold release characteristics and is superior to metal compound-coated stampers.
On the other hand, the fluorine polymer film is not fully adhesive to the substrate, or the stamper surface of nickel or the like, and it can come off the stamper surface owing to localized variation of applicable forces and also to thermal stresses with repeated shots for injection molding of resin disks. Because of this drawback, the film is not relied upon for the extension of stamper life.
It is an object of the present invention to provide a wear-resistant, long-life mold.
Another object of the invention is to provide a mold with reduced frictional resistance.
Still another object of the invention is to provide a stamper and other mold parts which exhibit good mold release characteristics with respect to the resin being molded and hence good durability.